Certain kinds of analytic procedures require the analysis of multiple fluid samples, where the samples have markedly different thermal characteristics, for example different heat capacities. A specific example is the MIGET by MMIMS (Multiple Inert Gas Elimination Technique by Micropore Membrane Inlet Mass Spectrometry) analysis, in which inert gas partial pressures are measured in two blood samples and one gas sample (Baumgardner J E, Choi I-C, Vonk-Noordegraaf A, Frasch H F, Neufeld G R, Marshall B E. Sequential VA/Q distributions in the normal rabbit by micropore membrane inlet mass spectrometry. J Appl Physiol 2000; 89:1699-1708). At the beginning of analysis, the blood and gas samples are at room temperature (typically 22° C.) and the samples must be heated, and analyzed at body temperature (typically 37.0° C.). Yet these blood and gas samples have very different heat capacities. The fluid samples flow past their individual sensors for measurement of the inert gas partial pressures in the samples. In addition to the different heat capacities of the samples, the optimal flow rate of the gas and blood samples is different. Despite these two different thermal characteristics (heat capacity and sample flow rate), both samples must be analyzed at an identical, and precise, temperature.
Thermal characteristics that might vary between multiple fluid samples include heat capacity (as in MIGET by MMIMS), sample flow rate (as in MIGET by MMIMS), sample volumes (for example multiple arterial blood gas samples where each sample has a different volume), and initial sample temperature (for example samples from different sources that all need to be analyzed at the same temperature). Additionally, multiple sensors used to analyze samples may vary in their thermal characteristics, and yet in some instances it may be desired to perform the analyses with each sensor at the same temperature.
In addition to the need for temperature control of multiple samples in analytic applications, it is sometimes also desired to carry out two or more fluid phase chemical reactions and maintain these parallel reactions at the same temperature. Possible differences in thermal characteristics between reactions include different reactant feed temperatures; different reactant feed flows; different volumes of reactants; and different specific heats of reaction. Despite these differences in thermal requirements of the reactions, it may be desired to carry out the parallel reactions at precisely the same temperature.
When analyses of multiple fluid samples are to be carried out at the same temperature, it is often desired to precisely regulate that temperature during the entire time it takes to make the measurements. For example, in MIGET by MMIMS, analysis of the inert gas partial pressures takes several minutes, and precise control of the analysis temperature to within 0.1° C. during this time can increase the accuracy of the inert gas measurements. Similarly, in multiple parallel fluid phase reactions, it may be desirable to precisely control the reaction temperature during the entire course of the reaction. For example, in the polymerase chain reaction (PCR), precise control of reaction temperature at 72° C. for approximately 20 seconds during the extension reaction may increase the overall efficiency of DNA sample doubling (Chiou J, Matsudaira P, Sonin A, Ehrlich D. A closed-cycle capillary polymerase chain reaction machine. Analytical Chemistry 2001; 73:2018-2021).
In addition to the requirement to maintain multiple samples at the same constant temperature for a period of time, it is sometimes desirable also to change the analysis temperature rapidly between sets of samples. For example, in both MIGET by MMIMS and arterial blood gas (ABG) analyses, different samples are often drawn from patients or subjects at different body temperatures, and it is highly desirable to be able to change the controlled analyzer temperature from one body temperature to another as these sample sets are processed sequentially. Similarly, for the purposes of carrying out multiple parallel reactions, it is sometimes desirable to rapidly change the reaction temperature from one controlled temperature to another, for example the rapid changes in temperature desired between the denaturing, annealing, and extension reactions of PCR (Nagai H, Murakami Y, Yokoyama K, Tamiya E. High throughput PCR in silicon based microchamber array. Biosensors and Bioelectronics 2001; 16:1015-1019).
Thus, in both analytical applications and in fluid phase reactor applications, there are sometimes multiple requirements for the overall process of temperature control: (1) provide for the temperature regulation of multiple fluid samples, sensors, or fluid phase reactions when the individual samples, sensors, or reactions have widely differing thermal characteristics; (2) provide temperature regulation that is highly precise, and uniform over a specified period of time; (3) provide temperature regulation for all of the samples, sensors, or reactions, that is highly precise, and uniform amongst the multiple samples, sensors, or reactions; and (4) provide for rapid and predictable changes in the controlled temperature. In the design of temperature controllers, these competing requirements often conflict. In particular, controllers that are capable of precise and uniform temperature regulation over time and amongst samples are generally not also adept at rapid temperature changes. Conversely, temperature controllers that can provide rapid temperature changes are often not precise and uniform. Prior art has therefore approached these problems in different ways.
One approach has been to place the samples, sensors, or reactants in a block of material that is highly thermally conductive, for example an aluminum heater block. For example, Shoder et. al. reported on the performance of 6 commercially available thermal cyclers for PCR, all based on the conductive block design (Schoder D, Schmalwieser A, Schauberger G, Kuhn M, Hoorfar J, Wagner M. Physical Characteristics of Six New Thermocyclers. Clinical Chemistry 2003; 49:960-963). Because of the high thermal conductivity, the block tends to be isothermal. Controlling the temperature of the samples within the block is then a relatively simple matter of controlling the block temperature. Because there are few restrictions on the size of the device used to measure block temperature, the block temperature can be measured with a highly accurate sensor such as a thermistor, or an integrated circuit type of sensor. Feedback control of block temperature requires only one control loop regulating the output of a block heater. In the conductive heater block approach, accuracy of temperature control is usually very good; also, samples that are uniform in their thermal characteristics will be uniformly controlled to the same temperature. This approach, however, has several disadvantages. First, if the samples have widely varying thermal characteristics, their temperatures will not always be uniform, because local variations within the block are not monitored or independently regulated. Second, the thermal mass of the block is usually substantially larger than the thermal mass of small liquid samples. The large thermal mass of the block makes it difficult to change sample temperature rapidly. When a rapid change in temperature is desired, such as step change to a new temperature, control algorithms such as PID (proportional-integral-derivative), which are well-known to those skilled in art, typically make a tradeoff between rapid changes versus overshoot of the target temperature. (Schoder D, Schmalwieser A, Schauberger G, Kuhn M, Hoorfar J, Wagner M. Physical Characteristics of Six New Thermocyclers. Clinical Chemistry 2003; 49:960-963).
A second approach to controlling the temperature of multiple samples, sensors, or reactions has been individual and independent heating of each sample. For example, Friedman and Meldrum reported a novel film resistor approach for thermal control of individual capillaries for PCR (Friedman N A, Meldrum D R. Capillary tube resistive thermal cycling. Analytical Chemistry 1998; 79:2997-3002). In this approach, the temperature of each sample, sensor, or reaction is independently measured, and used to control the output of an individually regulated heater. This approach easily accommodates multiple samples with widely varying thermal characteristics, because each sample is independently regulated. Also, the thermal mass of the individually heated parts is typically small, making it possible to change temperatures rapidly. This approach, however, has some disadvantages. For very small fluid samples, it introduces the complexity of measuring temperature in a very small sample. Temperature sensors amenable to miniaturization, such as thermocouples, do not provide accuracy comparable to larger sensors, such as thermistors. Also, it is often impractical to measure the fluid sample temperature directly, and a surrogate temperature (for example temperature on the surface of a capillary where the capillary contains the sample) is measured instead (Friedman N A, Meldrum D R. Capillary tube resistive thermal cycling. Analytical Chemistry 1998; 79:2997-3002). However, without the essentially isothermal temperature field provided by a conductive block, this can lead to errors in sample temperature measurement. As a result, individually controlling the temperatures of small fluid samples allows rapid changes in temperature, but does not usually result in the precision or uniformity (over time and between samples) of temperature control that is provided by a conductive block.
Certain kinds of applications, in particular the MIGET by MMIMS analysis, therefore present multiple performance requirements that are not completely satisfied by prior art. While prior art presents designs that meet these performance requirements individually, there is no prior art approach that meets all of these performance requirements.
A number of U.S. patents are directed to the general field of controlling the temperature of samples.
U.S. Pat. No. 6,730,883 teaches that earlier heater assemblies for carrying out PCR in discrete (i.e. non-flowing) samples in sample tubes did not provide uniform thermal contact with each sample tube cap, resulting in non-uniformity of temperature control between the samples, resulting in less efficiency of the PCR reactions. This patent teaches the use of a flexible heating cover assembly that provides uniform thermal contact to each sample tube cap. The device is preferably used in conjunction with a thermal heating block that holds the sample tubes. The thermal heating block teaches the use of various heater elements such as thermoelectric and resistive, and heat sinks such as forced convection and thermoelectric, but does not teach limitation of the samples to essentially a single plane positioned between a heat source and heat sink. The device also does not discuss the use of channels for flowing samples through the heater block.
U.S. Pat. No. 6,703,236 also teaches that in earlier thermal conductive blocks for discrete samples for the PCR reaction, non-uniformity of temperatures between samples was a problem that led to less efficiency. This patent teaches the use of a thermal block with heating provided by a resistive heater and cooling provided by flowing a liquid coolant through flow channels machined in the block. The cooling channels are interposed between the heater elements and the samples.
U.S. Pat. No. 6,692,700 teaches the use of large diameter leads to resistive heaters in microfluidic devices, to reduce unwanted heating of the leads as they pass through the device. This patent also teaches the use of thermoelectric chips to cool microfluidic devices.
U.S. Pat. No. 6,673,593 teaches the use of an integral semiconductor heater for applying heat in microfluidic devices.
U.S. Pat. No. 6,666,907 teaches the use of a thin film resistor in contact with a gas chromatography column where the resistor is used to directly heat the column, and the resistance is monitored to provide integral temperature sensing. The device provides a microfluidic approach to temperature programming for GC analysis.
U.S. Pat. No. 6,657,169 teaches that uniform temperature regulation of all samples of PCR is highly desirable, and teaches a conductive block for uniform heating of liquid samples. The patent teaches a thermal conductive block for heating PCR samples tubes, with resistive and thermoelectric heating elements and a natural convection heat sink, with the heaters positioned between the samples and the heat sink.
U.S. Pat. No. 6,579,345 teaches the direct heating of a capillary column for temperature programming, for gas chromatography. This patent teaches that requirements for rapid temperature changes conflict with requirements for precise temperature regulation, and teaches the use of a predictive, feed-forward control algorithm for use in conjunction with more traditional feedback control algorithms.
U.S. Pat. No. 6,558,947 teaches the use of special sleeves for holding PCR sample tubes, where each sleeve is individually heated, and each sleeve conducts heat to a heat sink. Each sample well is equipped with a temperature monitor, and the temperature of each sample tube is independently regulated.
U.S. Pat. No. 6,541,274 teaches the use of heat exchangers inserted into microfluidic fluid receptacles for controlling reaction temperatures.
U.S. Pat. No. 6,533,255 teaches the use of liquid metal for uniform temperature regulation of multiple samples, preferably used for PCR reactions.
U.S. Pat. No. 4,443,407 teaches a device for analyzing small blood samples at a fixed and controlled temperature of 37.0° C. The samples flow through a sample cell that is in thermal contact on both sides with conductive heater blocks, each maintained at 37.0° C. The heater blocks are heated with resistive heaters, and the blocks have several exposed surfaces that lose heat to the environment by natural convection.
U.S. Pat. No. 4,415,534 teaches a device for analyzing small blood samples at a fixed and controlled temperature of 37.0° C. The blood samples flow through a conductive measuring block, which contains the electrode sensors for various analyses. The conductive measuring block is surrounded by a conductive heat shield, with good thermal contact between the measuring block and heat shield at a conductive base member. Both the measuring block and the heat shield are maintained at 37.0° C. with heat supplied by a power transistor.